The invention relates generally to force measuring instruments and more particularly to an instrument for measuring multiple loads on full scale flight articles, structural test loading, static test items, any more particularly to an instrument for measuring the forces on wind tunnel models.
There are six components of the force and moment action on a model which are of interest to the designer in evaluating the flying qualities of an aircraft. These six components are well known by those skilled in the art as lift force, drag force, side force, pitching moment, yawing moment, and rolling moment. When determining the magnitude of these components acting on a scale model in a wind tunnel, certain design perimeters can be obtained which will apply to the full scale aircraft.
Prior art strain gage balances have been successfully utilized to measure the forces in wind tunnel models. The moments and forces acting on the model were usually resolved into three components of force and three components of moments but providing different members within the balance that were sensitive only to one or two components. Each of the members carry strain gages which were connected in combinations that formed Wheatstone bridge circuits. By appropriately connecting the strain gages, the resulting Wheatstone bridge circuit unbalances could be resolved into readings of the three components of force and three components of moment. All access to the model is by way of the sting support, having the balance attached to the upstream end. The balance is small enough to fit through the aft end of the model into a cavity within the model.
Thus it can be seen that all tubes, hoses, wires and such must compete within the balance or the small cross section of area available within the slim cavity of the models.
Wind tunnels require the use of six component load measuring devices to measure the load on wind tunnel models within 0.3 percent of maximum load accuracy. Prior art balances have been designed for a defined maximum load. The accuracy of these balances is based on percent of maximum load capacity. This results in large errors at low loads. The normal variation in axial force during wind tunnel testing is 10 to 1. This means that a large majority of testing is at 0.1 of maximum load capacity of the load measuring element. The result is data accuracies of ten times the required balance accuracy.
It is an object of the invention to provide a novel six component balance that can provide significantly increased accuracy when measuring loads varying from a low load to the maximum loads.